Environmental Engineering Reference
In-Depth Information
roots, the PAH concentrations were similar to the unplanted
areas. The authors hypothesized that the age of the roots had
a direct impact on the overall PAH removal potential, such
that older, more established shallower roots provided greater
PAH degradation.
Ouvrard et al. (2006) investigated the production of
organic-rich root exudates in the rhizosphere of plants
exposed to PAH-contaminated soil and the relation of these
exudates to the subsequent bioavailability of the PAHs for
microbes or plant uptake. The organic compounds released
by roots, such as malic acid, citric acid, and oxalic acid, can
provide for the possible absorption of PAHs and other
organic compounds. Ouvrard et al. (2006) determined that
the presence of model root exudates such as malic and
malonic acids was responsible for an increased removal of
the PAH phenanthrene from solution following linear
isotherms. They suggested that this enhanced soil sorption
was a consequence of the organic acids modifying the soil
aggregate structure to expose hidden sorption sites, rather
than acting as sorption sites themselves. The problem with
root exudates, however, is that they are readily metabolized
substrates for the heterotrophic rhizospheric populations,
and these exudates only become available if production is
greater than consumption.
Many studies cited here on the effect of the enhanced
degradation of PAHs in vegetated systems is due primarily
to the enhanced microbial populations associated with the
rhizosphere. However, other mechanisms associated with
the presence of plants have been offered. Gregory et al.
(2005) showed that organic matter released by plants can
sequester PAHs and PAH metabolites and that these
compounds become part of the natural cycle of humification
in planted areas. Although the release of root exudates
provides a labile substrate to support an active microbial
community, they also act to help form the humic fraction
of soils by decreasing larger fractions or increasing smaller
fractions. The first process would tend to increase PAH
degradation. The latter process would tend to decrease
PAH attenuation because it leads to the production of sorp-
tion sites to render PAHs less bioavailable.
The effect of rhizosphere bacteria on PAH-contaminated
soils in the unsaturated zone was investigated by Muratova
et al. (2003). They exposed alfalfa (
Medicago sativa
L.) and
a reed (
Phragmites australis
(Cav.) Trin. ex Steud.) to PAH-
contaminated soils in pots in the laboratory over a 2-year
period. Naphthalene was 8.71 mg/kg, and total PAHs were
79.80 mg/kg. Unplanted pots also were prepared. At the end
of 2 years of interaction, the unplanted control PAH
decreased from 79.80 to about 50 mg/kg PAHs. The planted
control PAH concentration decreased to about 30 mg/kg.
The authors attributed the increased loss of PAH to vegeta-
tion-induced microbial activity. Alfalfa plants had 1.3 times
more total soil bacteria, although in the reed no increase was
Fig. 13.7
The decrease in PAH concentrations following establish-
ment of poplar trees at a site of PAH-contaminated groundwater
(Modified from Widdowson et al. 2005a). One meter is equivalent to
3.2 ft.
including data for transpiration rates or estimates for
VPD
and contaminant loss.
13.3.2 Plant Transformation Reactions
One of the first studies to relate the root distribution of a tree
to the fate of PAHs in the subsurface was by Olson and
Fletcher (1999). They investigated some “volunteer plants,”
or those that grew naturally in a contaminated area, such as
mulberry, at a site of a former waste disposal basin. The soil
in which the plants were growing consisted of a soil
sludge
that contained PAHs. The largest roots were found in the
upper layer of soil
sludge to around 23 in. (60 cm), and the
PAH concentrations were no more than 20% of that charac-
teristic of the sludge in other areas. Between 23 and 39 in.
(60 and 100 cm) deep, which was characterized by smaller
Search WWH ::
Custom Search